1. Technical Field
The present invention relates to an image forming apparatus having an image forming engine which forms an image by forming a static latent image by irradiating light beam corresponding to image data onto the surface of an image carrying body charged uniformly, developing that image with toner, transferring the toner image directly or through an intermediate transfer body and fixing the transferred toner image and, to an output image density correction method thereof.
2. Related Art
Conventionally, an image forming apparatus employing this kind of electronic photographing method such as a copier, printer, facsimile, or multi-function machine thereof includes a charging portion, light beam scanning portion, developing portion, transferring portion and the like, disposed around a photo conductor drum as an image carrying body such that they oppose the peripheral surface of this photo conductor drum.
That is, the surface of the photo conductor drum is charged uniformly by applying a predetermined voltage to the charging portion, a static latent image is formed with a light beam from the light beam scanning portion, that image is developed by supplying toner in the developing portion, the toner image is transferred to an intermediate transfer body or the like by the transfer portion, and then that image is finally transferred to a paper.
The paper to which the image is transferred is subjected to fixing processing by the fixing portion during the transportation up to a discharge port.
In such a conventional image forming apparatus, a method for adjusting toner density in the developing portion and output image density with a single light beam sensor (density sensor) has been invented for reduction of manufacturing cost and reduction of the apparatus size. A specific example includes forming a toner density adjustment patch and an image density adjustment patch at a predetermined interval, determining a toner density and image density by measuring these patch densities with the light beam sensor, and adjusting the toner density and image density corresponding to this determination result.
To stabilize the image quality, the technologies described have been proposed.
A first related art aims at obtaining uniformity of image density and fixing property so as to maintain image quality in printing at a high image ratio in order to stabilize the image quality with a simple structure.
That is, the structure of this related art includes a portion which counts the amount of dots, a memory which stores a dot count value, and a control portion which controls toner development bias, and the bias voltage is raised when the dot count value exceeds a predetermined value.
As a consequence, the apparatus of this related art counts the amount of dots of an outputted image each time and increments the count value at the memory portion and when this value exceeds a value when the image quality begins to drop, the apparatus raises its development bias so as to suppress the quality drop in the density.
A second related art has proposed improvement with respect to the drop in solid density or the drop in character image quality due to the charge-up of the developing agent because the amount of toner discharged (developed) is insufficient when a number of low density images which are nearly blank sheets are outputted, and improvement with respect to the increase of running cost due to the acceleration of toner consumption pace within the developing machine in the case of a high image density (because of reduction in the charge of the toner).
That is, when the amount of discharge is insufficient, the amount of toner calculated under the discharge mode (by the adjustment control of the developing machine) is discharged so as to consume toner of more than a predetermined amount for a specified number of sheets. If the amount of discharge is large, toner is charged by operating the developing machine for a predetermined time.
A third related art has proposed maintaining a specified printing performance by securing the density by correcting the development bias, which method is used in the field of electronic photographing type high-speed printers because the low density appears after a printing operation at a low printing ratio or an intermittent printing operation.
That is, the correction is executed step by step corresponding to a detection result of the developing agent consumption rate in order to prevent the tone jump caused by correction.
However, in the above-described image forming apparatus, actual toner density or image density often largely diverges from a target even though a measured value by the aforementioned light beam sensor is near a target toner density or image density. The reason for this is due to dispersion in the amount of developing agent (MOS) on a developing roll, a degree of scattering of a patch image, or dispersion in the developing agent, photo conductor, intermediate transfer member or the like, and in any case, if these values largely diverge from their target values, a variety of troubles occur in image quality.
Particularly if the image density is higher than the target value, there is not only a problem concerning the image quality such as the generation of a ghost or transfer failure but also a problem that the life of a toner cartridge cannot be satisfied because the amount of consumed toner is large.
The correction disclosed in the first related art is not sufficient since the correction is not conducted when the number of dots is smaller than a number at which the quality drop begins. Further, the related art does not teach means to conduct correction when the image density is high.
Further, the second related art has no effect in keeping constant image density during normal operation because it intends to suppress toner charge-up or an influence at the time of low charge.
Additionally, the third related art has no effect in keeping constant image density during normal operation because it intends to suppress toner charge-up or an influence at the time of low charge.